Reporter gene technology is widely used to monitor cellular events associated with signal transduction and gene expression. Transcriptional regulation, coupled to the expression of a reporter gene is routinely used to monitor a wide variety of cellular events. To establish a reporter gene assay, the reporter gene is placed under the transcriptional control of a promoter or an enhancer with a minimal promoter. The reporter is inserted into a suitable plasmid vector typically containing a selectable marker that confers resistance to growth suppressing compounds, such as antibiotics. The vector DNA is introduced into cells using standard laboratory procedures. Addition of a suitable agonist will switch on the cell signalling pathway, leading to activation of a transcription factor and gene expression. A review of reporter gene technology is given by Naylor et al, in Biochem. Pharmacol., (1999), 58, 749-757.
A cell-based fluorescent gene reporter system has been described, the assay employing bacterial nitroreductase (NTR) and a cell permeable nitro-substituted quenched (or non-fluorescent) cyanine dye (shown as Compound (i)), which functions as a substrate for the enzyme (US 2003/0186348, Thomas, N. et al).
Cellular uptake of the substrate, by passive diffusion across the plasma membrane, was promoted through the use of ethyl ester groups to mask latent polar functionality. Intracellular cleavage of the ester groups by cellular hydrolases results in retention of the substrate inside live cells. Addition of the substrate to a cell that is expressing nitroreductase results in the reduction of the nitro group to the hydroxylamine with a concomitant increase in fluorescence emission. Depending on the structure of the quenched cyanine dye, the fluorescence emission from the product of the NTR reaction may be generated across a wide range of wavelengths, typically 500-900 nm. Emission at longer wavelengths is advantageous in avoiding background fluorescence and increasing sensitivity in biological systems.
Wild type nitroreductase expressed from a reporter construct is localised in the cytoplasm of the host cell (Spooner et al, Int. J. Cancer, (2001), 93, 123-30). To achieve a maximum signal output from the assay it is desirable to localise the substrate to the same cellular compartment as the reporter enzyme, i.e. within the cytoplasm of the host cell such that the substrate is available for activation by nitroreductase. The masking of hydrophilic groups on, or attached to, the substrate molecule can generate membrane permeable compounds. Furthermore, the masking group can be designed to cleave from the substrate within the cell to generate the substrate intracellularly, preferably within the cytoplasm of the cell. Masking strategies to enable delivery of nitro-substituted cyanine dyes relatively uniformly to the cell cytoplasm have not proved to be entirely successful. A study of the localisation of cell permeant quenched cyanine dye (Cy-Q) derivatives within cells using fluorescence microscopy, has shown the localisation of some of the substrate to internal cell membranes and organelles, predominantly the mitochondria of the cell. Accumulation of lipophilic, cationic nitro-substituted cyanine dye substrates within mitochondria is accompanied by an increase in fluorescence of the probe and this accumulation results in an increase in background fluorescence in NTR assays. Thus, there is a need for new and improved reagents for use as NTR substrates that display lower background fluorescence, improved fluorescence signal and cellular distribution.
Squarylium (squaraine) dyes are a class of dyes that have overall electrical neutrality; an example is shown as Compound (ii).
Nitro-substituted squaraine dyes are known from EP 645680 (Bugner D., et al) as near infra-red absorbing additives for use in electrophotographic imaging processes. PCT Application No. WO97/40104 (Hamilton, A. L. et al) discloses squaraine dyes and adducts of squaraine dyes with biological molecules such as peptides, proteins and nucleotides. The dyes may be substituted by electron donating and electron withdrawing substituents, for example nitro; however, the fluorescence properties of the nitro-substituted dyes are not disclosed. The present inventors have now discovered that nitro-group-containing quenched squaraine dyes are effective substrates for nitroreductase through reduction of the nitro group, resulting in a change in an optical property, preferably a change in fluorescence emission, of the squaraine dye. Use of nitro-substituted squaraine dyes in assays for determining nitroreductase activity results in greater sensitivity and lower background fluorescence than in assays that employ conventional NTR substrates.